This invention is directed to interferometric measurement systems for measuring distances.
Laser interferometers are widely used to measure a distance to a sample surface. Laser interferometers operate by splitting a source of coherent radiation into two beams, an object beam and a reference beam, using, for example, a beam splitter. The reference beam is directed to a reference mirror which is located at a known distance from the beamsplitter. The object beam is directed to a sample surface or target which is located at some unknown distance from the beam splitter. The object beam is reflected from the sample surface, and is recombined with the reference beam at the beamsplitter. The object beam and reference beam constructively or destructively interfere, depending on the relative phase of the object beam compared to the reference beam. A detector measures the intensity of the interference light, which is related to the phase difference between the object beam and the reference beam, which is, in turn, related to the difference in the object path length from the beam splitter to the sample surface compared to the reference path from the beam splitter to the reference mirror. “Absolute” laser interferometer systems are known that use multiple wavelengths, and analyze the respective phase differences corresponding to the respective wavelengths to eliminate uncertainty regarding which specific wavelength or period of the cyclic interference light corresponds to a current measurement.